Prior-art generators comprise at least one first coil, one second coil and one third coil wound respectively about a first, second and third winding axes, the first and second and third winding axes being mutually orthogonal and secant at a common point corresponding to the central point, the coil or coils wound around a same winding axis being laid out along this winding axis so that a plateau of intensity of the magnetic field generated by this coil or these coils occurs at a central point when this coil or these coils are crossed by a same current.
When the plateau of intensity of the magnetic fields generated along each of the three orthogonal axes occurs at a central point, it is said that the magnetic field is homogenous. More specifically, a homogenous magnetic field is a field for which at least the first derivative is zero at the level of the central point. When only the first derivative is zero, it is called a second-order homogenous field because the first non-zero derivative in the Taylor series decomposition of the magnetic field at the central point is the one corresponding to the second derivative.
Here below, a magnetic field is said to be Nth-order homogenous when the first non-zero derivative in the Taylor series decomposition of the magnetic field at the central point is the one corresponding to the (N−1)th derivative.
Here below, the term “Nth-order generator” also designates a generator of a Nth-order homogenous magnetic field.
When the generator generates a homogenous magnetic field in all three orthogonal directions at the same central point, the magnetic field is homogenous within a sphere centered on this central point.
At present, the manufacturing of these generators is complicated because they are made by coiling wires into grooves made in a sphere made of electrically insulating materials.
The prior art is also known from DE10148288A1, JP2005294537A, JP2002232182A, EP2259081A1, EP1873543A1, and EP1481637A1.
This prior art describes a layout of coils that are more simple to make but for which the magnetic field is not as homogenous as the one generated by more complex generators. Furthermore, their space requirement is generally fairly great.